This invention relates to corrosion proof terminals for solid or stranded aluminum wire.
The problems associated with copper-aluminum terminations have long been recognized. Aluminum, upon exposure to air, forms a non-conductive, mechanically hard outer layer of aluminum oxide. The formation of the oxide layer is self-limiting. Thus, unlike the ferrous metals, progressive corrosion of the aluminum does not occur. The non-conductive nature of the oxide layer prohibits electrical conductance even when seemingly intimate contact has been established. A standard method of overcoming this non-conductance is to place a grease-abrasive mixture in intimate contact with the aluminum. When the terminal is crimped, the abrasive material cuts through the oxide film establishing contact. The grease prevents reoxidation. This method is undesirable in oil-filled transformers when the grease and abrasive may contaminate the oil. With stranded aluminum wire such a method does not insure good interstrand contact.
Other solutions to the oxide problem are tin and copper clad aluminum wire. A good contact material is plated or clad directly onto the wire, thereby by-passing the oxide formation. Such wire is expensive.
Creep or cold flow is another problem associated with aluminum. Aluminum has no ultimate rigidity. Thus, it will deform away from applied stress. The rate of creep increases with temperature. In a pressure connection the creep away from the stressed area causes high resistance and heating. The heating causes acceleration of the creep. The final result is failure of the joint.
The creep problem has been overcome by maintaining a constant stress on the joint by residual pressure. This has been done with spring washers in bolted connections and with a C-member having spring characteristics. The residual pressure in the C-member tends to follow-up on the wire as it creeps, thereby maintaining a continuous stress on the contact area. In crimped connections, the creep problem is minimized by completely confining the wire material in the wire barrel.
Electrolytic or galvanic corrosion is another problem with copper-aluminum terminations. Since the electrical potentials of the two metals differ significantly, exposure to moisture containing an electrolyte establishes a galvanic cell with resulting current flow and ion exchange. Plating of copper with a metal having an electrolytic potential more similar to aluminum reduces the effect of the electrolytic action. Grease compounds also provide protection by preventing the corrosion media from contacting the coupled materials. The most effective way of removing electrolytic corrosion is to isolate the termination from the environment. This may be achieved by insulating the termination.